The present invention relates to an electrophotographic printing original plate and an electrophotographic printing plate making process of making a printing plate by use of the electrophotographic original printing plate. More particularly, the present invention relates to an electrophotographic printing original plate comprising an electroconductive support material and an electrophotographic photosensitive layer formed thereon, which electrophotographic photosensitive layer comprises (i) a charge generation layer essentially consisting of a particular azo pigment serving as charge generating material, and (ii) a charge transport layer consisting essentially of a charge transporting material and an alkali-soluble resin, and the present invention also relates to an electrophotographic printing plate making process comprising the steps of charging uniformly the above printing original plate, exposing the charged printing original plate to optical images to form latent electrostatic images thereon, developing the latent electrostatic images with toner to visible toner images, fixing the toner images to the printing original plate, and removing the electrophotographic photosensitive layer portions corresponding to the non-image areas of the printing original plate by dissolving the non-image areas in a dissolving liquid, thereby making a printing plate.
As conventional lithographic printing plates, printing plates using photosensitive resins and printing plates using silver halide photosensitive materials are known.
A lithographic printing plate using a photosensitive resin has a high printing durability, but has the shortcomings that the photosensitivity is low, direct plate making cannot be done, a positive or negative film must be prepared from the original, using a silver halide film, therefore a large-scale apparatus is necessary and the plate making process is very time consuming.
A printing plate for the silver salt diffusion transfer process and a printing plate for the tanning development allow direct plate making. However, they have the shortcomings that the printing durability is poor and printing cost per sheet is very high.
As the printing plate for direct plate making utilizing electrophotography, for instance, printing plates of a zinc oxide-resin dispersion type are known as disclosed in Japanese Patent Publications No. 47-47610, No. 48-40002, No. 48-18325, No. 51-15766 and No. 51-25761. A printing plate of this type is treated with an acidic aqueous solution containing, for instance, a ferrocyanide, in order to make the non-image areas of the printing plate hydrophilic after the formation of toner images by electrophotography on the printing plate. The thus prepared printing plate has a low printing durability of about 5,000 to 10,000 sheets, since the photosensitive layer and the electroconductive layer of the printing plate peel off the support material of the printing plate due to the mechanical pressure applied thereto and penetration of dampening water or solution into those layers during the course of printing, so that the hydrophilic surface layer of the printing plate is damaged.
Further, in such a zinc oxide--resin dispersion type printing plate, dye sensitization is carried out in order to cause the printing plate to have photosensitivity in the visible range. This sensitization, however, is not sufficient for practical use in a long wave range of 600 nm or more. Therefore, inexpensive low power He-Ne laser and semi-conductor laser cannot be employed for formation of latent images on this printing plate.
For instance, Japanese Patent Publications No. 37-17162, No. 38-7758, No. 46-39405, and Japanese Laid-open Patent Applications No. 52-2437, No. 56-107246, No. 55-105254, No. 55-153948, No. 55-16125, No. 57-147656, No. 56-146145 and No. 57-161863 disclose electrophotographic printing plates comprising a hydrophilic electroconductive support material such as a grained aluminum plate, a layer comprising an organic photoconductive compound and a alkali-soluble resin formed on the support material, or a layer comprising a charge generating pigment layer such as a phthalocyanine-type pigment which is dispersed in an alkali-soluble resin, or a more sensitized layer comprising a charge generating pigment such as a phthalocyanine-type pigment which is dispersed in an alkali-soluble resin by addition thereto of by an electron acceptor or an electron donor. The printing plates of this kind are prepared by removing the photosensitive layer in the non-image areas using an alkaline solution after electrophotographic formation of toner images on the printing plates. These printing plates have the advantage over other conventional printing plates that they have high printing durability, but have the shortcomings that the photosensitivity is low and therefore the plate making speed is significantly low, so that a high power light source is necessary for plate making.